Detection and eradication of minimal residual malignant disease is a major goal in cancer therapeutics. Nowhere is this more important than following hematopoietic cell transplantation (HCT) where the vast majority of patients are rendered to a state of minimal disease yet a significant percentage of patients will ultimately relapse. We have developed a novel animal model system to study both the fate, distribution and survival of minimal numbers of cancer cells and track immunological effector cell populations capable of controlling disease. To accomplish this goal we have introduced the luciferase (luc) gene into either the tumor or lymphoid populations of cells which can be tracked in vivo by light emission. Novel fusion genes have been constructed combining the green or yellow fluorescent proteins (gfp or yfp) which can be utilized for cell sorting and immunohistochemistry with that of luc which allows for in vivo imaging. Using this model system as few as 1,000 cells can be visualized, quantitated and tracked in living animals non-invasively. We have demonstrated that the amount of light emission is a quantitative measurement of tumor burden. The temporal and spatial assessment of disease progression and metastasis as well as response to therapy can be quantitatively, non-invasively assessed in real time. We will utilize models of both myeloablative and nonmyeloablative HCT to evaluate the role of these therapies in controlling minimal residual disease and affecting the development of graft vs host disease (GVHD). Lymphoid populations of cells will be evaluated for their ability to control minimal residual disease, suppress GVHD and enhance donor-specific engraftment. The gfp/luc fusion gene will be introduced into lymphoid effector cell populations to evaluate cell trafficking, survival and recovery of functionally active effector cells at sites of tumor or GVHD. The role, impact on T cell function and trafficking and in vivo fate of CD4+CD25 +T cells will be explored based upon our recent observations that these regulatory T cells are capable of suppressing GVHD while not impacting graft vs. tumor activity. Effector cells from selected knock-out strains and animals deficient in key molecules will be utilized to explore mechanisms of these diverse biological effects. These studies will provide important biological insights into the function of T cells in the control of malignant disease, suppression of GVHD and facilitation of engraftment. These insights will be utilized to develop strategies translatable to the clinic to improve outcomes for patients with a broad range of malignant conditions.